Excited State Populations of a Residual Hydrogen Impurity in a Helium Afterglow Plasma

Time-resolved absolute intensity measurements have been made
on the Balmer series lines of a residual hydrogen impurity in a hollowcathode
helium afterglow plasma. From these measurements, the excited
state populations were calculated and revealed the existence of population
inversions in the excited states of hydrogen. In the hydrogen
afterglow the maximum average state population shifted upwards from
n = 3 at the beginning of the afterglow to progressively higher states
until in the late afterglow the maximum population was at n = 6. The
effects of the helium metastables, both atomic and molecular, were
shown to offer no detectable contribution to the observed inversions.
Comparison with a theoretical model for a recombining pulsed hydrogen
plasma for which inversions had been predicted are favorable and
recomb ~nation processes are assumed responsible for the inversions in
the lower discrete levels in the hydrogen impurity.